linux-kernel - Re: [PATCH 0/3] virtio-net: inline header support

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Date:	Tue, 09 Oct 2012 15:29:09 +1030
From:	Rusty Russell <rusty@...tcorp.com.au>
To:	Paolo Bonzini <pbonzini@...hat.com>
Cc:	kvm@...r.kernel.org, "Michael S. Tsirkin" <mst@...hat.com>,
	netdev@...r.kernel.org, linux-kernel@...r.kernel.org,
	Sasha Levin <levinsasha928@...il.com>,
	virtualization@...ts.linux-foundation.org,
	Thomas Lendacky <tahm@...ux.vnet.ibm.com>, avi@...hat.com
Subject: Re: [PATCH 0/3] virtio-net: inline header support

Paolo Bonzini <pbonzini@...hat.com> writes:
> Il 05/10/2012 07:43, Rusty Russell ha scritto:
>> That's good.  But virtio_blk's scsi command is insoluble AFAICT.  As I
>> said to Anthony, the best rules are "always" and "never", so I'd really
>> rather not have to grandfather that in.
>
> It is, but we can add a rule that if the (transport) flag
> VIRTIO_RING_F_ANY_HEADER_SG is set, the cdb field is always 32 bytes in
> virtio-blk.

Could we do that?  It's the cmd length I'm concerned about; is it always
32 in practice for some reason?

Currently qemu does:

    struct sg_io_hdr hdr;
    memset(&hdr, 0, sizeof(struct sg_io_hdr));
    hdr.interface_id = 'S';
    hdr.cmd_len = req->elem.out_sg[1].iov_len;
    hdr.cmdp = req->elem.out_sg[1].iov_base;
    hdr.dxfer_len = 0;

If it's a command which expects more output data, there's no way to
guess where the boundary is between that command and the data.

Cheers,
Rusty.
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